Blue Grotto, Capri

Capri
Italy

Concierge.com's insider take:

No trip to Capri is complete without a visit to the island's most famous natural attraction, a partially submerged rock cave where refracted sunlight turns the water and walls a luminous blue. The cave was formed naturally, but Romans carved out a small landing stage and nymphaeum (a temple consecrated to the nymphs) at the back of the cave, with a tunnel that some say once reached all the way to Villa Damecuta, one of Tiberius's 12 villas, far above. In summer, motorboats leave Marina Grande for the Grotto every few minutes (you'll pay around €10 euros/$13 for a round trip). Once you arrive, you'll be decanted in groups of three into tiny rowboats (and must pull your wallet out yet again—the ride costs another €10). The boats are small enough to make it under the low lip of the cave, which is sometimes impossible to enter in rough seas. In theory, you can swim in, but the fearsome rowboat operators don't look kindly on this, and it's only really advisable when they're not around—before 9:30 am or just before sunset.